BKR5000 Portable Radio User’s Manual
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APCO Project 25 Digital Squelch Control
Network Access Codes (NACs) provide the digital equivalent of analog
sub-audible signaling (CTCSS/CDCSS) allowing a group of radios to be
selectively called within a system.
Users in the same area (using the same NAC) can be further divided
into Talk Groups, with each group having its own Talk Group ID (TGID).
Group Calls are made by designating both the users’ NAC and TGID.
Each radio also has an individual P25 unit ID. A Unit-to-Unit call
contains the addressee’s NAC, and uses the addressee’s P25 unit ID
instead of the TGID.
When operating in Digital Mode, each channel can be programmed to
use either Normal squelch or Selective squelch.
Normal squelch is used to mimic analog operation. Signals are only
qualified with the programmed NAC. TGIDs and P25 Unit IDs are
ignored. Each digital channel is programmed with a receive NAC and a
transmit NAC. When an incoming signal’s NAC matches the channel’s
programmed receive NAC, the radio unmutes. The default NAC is 0659
($293 hex).
The digital equivalent of carrier squelch is achieved by programming
the receive NAC = $F7E (3966 decimal). The radio will unmute when a
digital signal with any NAC is detected. The $F7E (3966 decimal) NAC
is reserved for receivers and is not allowed as a transmit NAC.
Selective squelch is used for processing Group Calls and Unit-to-
Unit Calls. TGIDs are assigned on a per-channel basis. Users can be
separated into Talk Groups with each group having its own TGID. Then,
on channels programmed for Selective squelch, the incoming signal’s
NAC and TGID must match the channels programmed receive NAC
and TGID for the radio to unmute. The default TGID is 1.
The TGID value 65535 ($FFFF hex) is used to effect an “All Call”. If the
radio receives a signal with a matching NAC and the TGID = 65535
($FFFF hex), it will unmute. Also, if the radio’s programmed TGID is
65535 ($FFFF hex), it will open on any signal with a matching NAC,
ignoring the incoming TGID. A TGID = 0 means “no one”. If the radio
is programmed with the TGID = 0, it will accept incoming group calls
containing the “All Call” TGID, and correctly addressed Unit-to-Unit
calls only.
